Conventionally, a three-way valve can comprise a pressure-tight housing, a valve seat, a closing member moveable in the valve seat and an operating member moveable in the housing through a shaft seal.
The housing is formed with three connecting channels and the valve seat has associated connecting passages. The closing member has a metering passage by which two of the connecting channels are alternately connectable with a third of the connecting channels to form a complete path through the three-way valve and the closing member seals the connecting channels from each other in an additional control position.
A ball valve with a valve seat made from an elastomeric material of this type is shown in U.K. Patent Application No. 21 37 735. The different control positions can be set by rotation of the ball valve. To determine which of the connecting channels are connected with each other an arbitrary decision must be made which correlates the position of the handle to the flow paths. Thus there is a danger of erroneous operation because the handle can be incorrectly mounted in the valve or because the user may not know the arbitrary decision which has been made.
In the known three-way valves it is also disadvantageous that the sealing surfaces on the valve ball are comparatively small and a sufficient service life is not guaranteed especially in corrosive and/or abrasive media. Leaks can lead to considerable problems when the three-way valve is used for control or metering of media which should not come into contact with one another.